[Corvette95]

Sounds like ( and truest no offense) but a shotgun approach to this. Maybe former owner, maybe a little bit of current owner, but a FSM and patient will go a long way. Now, sounds like you are past repairing current unit, you say C68 but do you really mean problems with C68 control head or the programmer? I have quite a bit of experience with both. Corvette did change and update the programmer in 90. It was due to the controller losing its position or calibration when battery power was removed or the unit was removed and reinstalled and the programmer could not find 75 degrees setting (middle temp). PSA ALWAYS set your temp switch to 75 when parking your car for any time, I do it every time , to prevent the rubber gasket surrounding the vent shaft not to sit and get stuck in either hot or cold, setting it to 75 cures that issue . Also, it is the default setting when battery power is removed from car and reconnected. The old programmer if not shot, you can remove all battery power from car. Remove the resistor panel on engine side of heater box, pull out the resistor housing and GENTLY move the blend door to mid way. ALSO spray liberty the doors runner flap with standard WD-40 (recommended maintenance-this problem or not) after reassembly (you can leave the resistor cover off outside blower housing for a visual check) cut a/c reconnect everything . Controller will move back and forth, sometimes for a long time (this is what the new design fixes) and sooner or later (15 minutes) your blend door hopefully self aligns, if not repeat this procedure. Once (and unfortunately if) it calibrates itself you are good to go but it will happen again if these directions are not followed. It?s a 90 only thing. I know this works most of the time but can be time consuming. It?s buried in the 90 FSM supplement but was shown amd proven to my by Gordon Killebrew. Moral of the story in absence of other issues , give the programmer as much time as it needs to find its zero (75 degrees center). Good luck hope this helps.